Two-coat single cure powder coating

ABSTRACT

Methods and systems for coating metal substrates are provided. The methods and systems include sequential application of low flow and high flow powder coatings followed by a single heating step to provide a cured coating. The methods and systems include a marker that allows coating uniformity to be monitored and assessed during application. The described methods provide coatings with optimal surface smoothness and edge coverage.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation-in-part of U.S. application Ser. No.14/019,041, filed 5 Sep. 2013, which is a continuation-in-part ofPCT/US/2012/070347, filed 18 Dec. 2012, which claims priority from U.S.Provisional Application Ser. No. 61/642,578, filed 4 May 2012 and U.S.Provisional Application Ser. No. 61/613,647, filed 21 Mar. 2012.

BACKGROUND

Powder coatings are solvent-free, 100% solids coating systems that havebeen used as low VOC alternatives to traditional liquid coatings andpaints.

Powder coating of metal parts is a common practice. It is difficult,however, to coat certain parts of a metal substrate, including edges andcorners, for example, to obtain a uniform coating using typical powdercoating processes. Consequently, edge corrosion is a common problem.Typically, when powder coatings are applied to metal parts, a low-flowcoating which provides good edge coverage is used. However, suchcoatings have a tendency to produce wavy surfaces characterized asorange peel, or surfaces with raised grains, i.e. surfaces with lowsmoothness. On the other hand, when flow is increased to provide greatersmoothness, edge coverage thins, and may fail altogether; leaving metalparts prone to edge corrosion. Conventional systems that attempt tocombine flow characteristics with increased surface smoothness typicallyrequire multiple application and heating steps, leading to processinefficiency and delay.

From the foregoing, it will be appreciated that there is a need foreffective powder coating of metal parts, where multiple cure cycles areeliminated, and where the coating demonstrates excellent performancecharacteristics, such as excellent corrosion protection, including atthe edges, and optimal surface smoothness.

SUMMARY

The invention described herein includes methods for coating metalsubstrates using one or more powder compositions. In an embodiment, themethods include providing a metal substrate and applying a first powdercoating on the substrate, where the first powder coating has flow of nomore than about 40 mm. A second powder coating is then applied on thefirst powder coating, where the second powder coating has flow of atleast about 40 mm. The two coatings are then heated in a single step toproduce a coating with good corrosion resistance, including at theedges, and surface smoothness.

In another embodiment, the present invention includes systems forcoating a metal substrate. The system includes a first powdercomposition with flow of no more than about 40 mm, and a second powdercomposition with flow of at least about 40 mm. When the first and secondpowder compositions are sequentially applied to the metal substrate andheated in a single step, the resultant cured coating has optimalcorrosion resistance and surface smoothness.

In another embodiment, the present invention includes methods forcoating a metal substrate, where the methods include providing at leasta first powder composition with flow of no more than about 40 mm, andoptionally, providing at least a second powder composition with flow ofat least about 40 mm. The methods further include instructions forcoating a metal substrate with at least the first composition, followedby coating with a second composition, and, in a single step, heating thetwo compositions to form a cured coating.

In yet another embodiment, the present invention includes methods andsystems for coating a metal substrate, where the methods and systemsinclude providing at least a first powder composition, which includes amarker, and optionally, providing at least a second powder composition.The methods or systems include instructions for coating a metalsubstrate with at least a first coating, wherein the presence of themarker in the first powder composition allows monitoring of theapplication of the second powder composition. The details of one or moreembodiments and aspects of the invention are set forth below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and from the claims.

Selected Definitions

Unless otherwise specified, the following terms as used herein have themeanings provided below.

The term “on”, when used in the context of a coating applied on asurface or substrate, includes both coatings applied directly orindirectly to the surface or substrate. Thus, for example, a coatingapplied to a primer layer overlying a substrate constitutes a coatingapplied on the substrate. Additionally, the term “metal substrate,” asused herein refers to substrates that are untreated, unprimed, orclean-blasted, and also to surfaces that have been primed or pretreatedby various methods known to those of skill in the art.

As used herein, the term “flow” refers to the relative flow-out of apowder composition on heating. Flow may be measured according to theprocedure described in ASTM Method D4242-07 (2013) (Test Method forInclined Plate Flow for Thermosetting Coating Powders).

The term “smoothness”, as used herein, refers to the specular gloss orlight reflectance from a powder-coated surface. It is typically obtainedby comparing the specular reflectance from a coated sample to thespecular reflectance from a black glass standard. As used herein,smoothness may be expressed by any means known to those of skill in thepowder coating art, including visual standards developed by the PowderCoating Institute (PCI Technical Brief No. 20). Under this standard, avisual scale of ten powder-coated panels, graded from 1 (highroughness/orange peel) to 10 (very smooth, high gloss finish) is used.To determine relative smoothness, a powder-coated sample is visuallycompared with the standard panels, and a smoothness grade is assigned byjudging which standard panel is closest to the sample.

In the alternative, surface smoothness may be expressed as 20-degree or60-degree gloss measured using ASTM Method D523-14 (Standard Test Methodfor Specular Gloss).

Additionally, smoothness may be assessed by monitoring the distinctnessof the image (DOI), where the reflection of a powder-coated sample ineach of the 10 PCI test panels is photographed, and the speed of a beamof light reflected from the surface is measured by a special instrument.Surfaces that reflect an image perfectly have DOI value of 100, whilesurfaces with little or no image clarity have DOI value of 0. The methodused to determine smoothness will typically depend on the ultimate enduse for the powder-coated substrate.

The term “curing,” as used herein, refers to a step of heating a powdercomposition to a temperature at which it begins to melt and flow. Theviscosity of the coating composition increases with temperature, ascrosslinking increases.

As used herein, the term “edge coverage” refers to the degree to which apowder coating covers the edges or corners of a substrate. It ismeasured using the procedure described in ASTM Method D2967-7 (2013)(Standard Test Method for Corner Coverage of Powder Coatings), modifiedby spraying the substrate (a square test bar) with the powder coatingcomposition rather than dipping the substrate in a fluidized bed. Edgecoverage is the ratio of the thickness of the coating at the edges ofthe test bar to the thickness of the coating on the face of the testbar, expressed as a percentage, where the face coverage refers to thethickness of the coating applied to each of the planar surfaces of thetest bar. The term “edge coverage” is used interchangeably with the term“corner coverage.” Edge coverage is typically validated by cabinetcorrosion testing.

The term “marker,” as used herein, refers to any chemical or physicalentity or component that can be included in a powder coating compositionand detected by physical or chemical means during powder application toa substrate. Physical means of detection include viewing with the nakedeye, under specific illumination conditions, with specialized viewingequipment or eyewear, and the like. Chemical means of detection includechemical reaction of the marker with other components in the powdercoating that can produce a visible or detectable change in the coating,typically a change in color.

The term “color change,” as used herein, refers to a color differencebetween the powder composition as applied and the color of the coatingafter the single heating step, wherein the difference in color isassessed using the L*a*b* scale.

The term “critical color match,” as used herein, refers to a colorsimilarity between the color of an uncoated substrate and a subsequentlyapplied coating, or a first coating applied to a substrate and asubsequently applied coating, wherein the similarity in color isassessed using the L*a*b* scale.

Unless otherwise indicated, the term “polymer” includes bothhomopolymers and copolymers (i.e., polymers of two or more differentmonomers).

The term “comprises” and variations thereof do not have a limitingmeaning where these terms appear in the description and claims.

The terms “preferred” and “preferably”, refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the invention.

As used herein, “a,” “an,” “the,” “at least one,” and “one or more” areused interchangeably. Thus, for example, a coating composition thatcomprises “an” additive can be interpreted to mean that the coatingcomposition includes “one or more” additives.

Also herein, the recitations of numerical ranges by endpoints includeall numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2,2.75, 3, 3.80, 4, 5, etc.). Furthermore, disclosure of a range includesdisclosure of all subranges included within the broader range (e.g., 1to 5 discloses 1 to 4, 1.5 to 4.5, 1 to 2, etc.).

DETAILED DESCRIPTION

Embodiments of the invention described herein include methods andsystems for powder coating a metal substrate, including at the edges.The methods include steps for applying at least a first powdercomposition to a substrate, and applying at least a second powdercomposition over the first composition. The methods further includeheating the first and second powder compositions in a single step toobtain a coated article with acceptable, preferably optimal edgecoverage and smoothness.

Accordingly, in some embodiments, the present invention provides methodsor systems for coating a substrate, including at the edges, with aprocess that uses a single heating step. Multiple application andheating cycles are thereby eliminated, resulting in a more efficientprocess. Moreover, as the methods described herein provide optimalprotection for the edges of substrates, mechanical methods to round offedges prior to coating are no longer necessary. Therefore, the methodsdescribed herein reduce the time, energy and cost of powder coating asubstrate, including at the edges, without compromising corrosionresistance or surface smoothness of the coating.

In an embodiment, the methods described herein include applying at leasta first powder composition to a substrate, such as a substrate withsharp edges, for example. The first powder composition is a fusiblecomposition that melts on application of heat to form a coating film.The powder is applied using methods known to those of skill in the art,such as, for example, electrostatic spray methods, to a film thicknessof about 10 to about 100 microns, preferably 25 to 75 microns. In anaspect, the first powder composition is applied to either the clean(i.e. unprimed) or pretreated surface of a metal substrate, i.e. thefirst powder composition may be applied to a metal surface that isunprimed, primed, that has been clean-blasted, or a surface that hasbeen pretreated by various methods known to those of skill in the art.

In an embodiment, the method described herein includes applying at leasta second powder composition to a substrate after at least a first powdercomposition has been applied. In an aspect, the second powdercomposition is a fusible composition that melts on application of heatto form a coating film, and may have the same chemical composition asthe first composition (i.e. the composition includes the same binderresin as the first composition, but different additives, pigments, andthe like), or it may be different (i.e. the composition includes adifferent binder resin than the first composition). The second powdercomposition is applied using methods known to those of skill in the art,such as, for example, electrostatic spray methods. The second powdercomposition is preferably applied over a coating of the first powdercomposition, to a film build of 10 to 100 microns, preferably 25 to 75microns. The total thickness of the film formed by the first and secondpowder compositions may be about 20 to 200 microns, preferably 50 to 150microns (approx. 3.0 to 6.0 mil).

On heating, the particles of the first and second powder compositionmelt and flow out to form the ultimate coating. Without limiting totheory, particles of the first powder composition melt to a low flowliquid that remains where deposited, including on the edges of thesubstrate, and forms the metal interface. Similarly, particles of thesecond powder composition, deposited on the first composition, melt to ahigh flow liquid that levels out over the surface of the firstcomposition to form a smooth coating and forms the air interface.

It is desirable to apply the first and second coating compositions atthe same film build. However, without limiting to theory, it is believedthat pretreatment or the presence of dirt, organic residues, and thelike on a substrate surface produce an uneven surface. Therefore, thefirst coating composition is applied at a greater thickness that thanthe second coating composition to account for the altered profile of thesubstrate. For example, where a total film build of 90 micron (approx.3.5 mil) is desired, the first coating composition is applied at a filmbuild of about 50 micron to account for the substrate profile, and thesecond coating composition is applied at a film build of 40 micron.

In an embodiment, the methods described herein include applying at leasta second powder composition after at least a first powder compositionhas been applied on the metal substrate. In an aspect, the second powdercomposition is applied such that a uniform coating of the second powdercomposition will substantially cover the entire first coating, i.e.,leave little to no part of the first composition exposed. Accordingly,in a preferred aspect, the first powder composition includes a markervisible during the application process, wherein the regularity oruniformity of the second powder coating may be assessed by monitoringthe marker during application. For example, where the marker is aUV-sensitive component, the substrate with the first powder alreadyapplied may be illuminated with a black light. When the second powdercoating is applied over the first, the illuminated marker will identifyany parts of the first coating that remain exposed and thereby alert theapplicator that the second powder coating needs reapplication orreinforcement. Alternatively, the marker may be included in the secondcoating composition, such that when the second powder composition isapplied, the illuminated marker will identify areas where the firstcoating is still exposed.

In an embodiment, the methods described herein include applying at leasta second powder composition after at least a first powder compositionhas been applied on the metal substrate. In an aspect, the second powdercomposition is applied such that a uniform coating of the second powdercomposition will substantially cover the entire first coating, i.e.leave little to no part of the first composition exposed. Accordingly,in an aspect, the second powder composition changes color during thecure process, and the regularity or uniformity of the second powdercoating may be assessed by monitoring the color change of thecomposition after the single heating step. For example, when the secondpowder composition is applied over the first, the change in color of thecoating when cured will identify any parts of the first coating thatremain exposed and thereby alert the applicator that the second powdercoating needs reapplication or reinforcement.

The first or second powder compositions may optionally be colored withdyes or pigments. Various organic or inorganic coloring pigments may beused in the present invention. Suitable coloring pigments includetitanium dioxide (TiO₂), carbon black, red iron oxide, yellow ironoxide, raw umber, phthalocyanine blue, phthalocyanine green, naphtholred, toluidine red, various organic yellows, various organic reds,carbazole violet, DPP reds, DPP yellows, and quinacridones. If desired,processed coloring pigments, such as pigments that have been coated withpolymeric materials may be used. Suitable such pigments include SURPASSproducts from Sun Chemical.

In an embodiment, the first and/or second powder composition(s)include(s) one or more pigments that are a first color when applied, andchange to a second (different) color on curing. Suitable pigments ofthis type include pigments that undergo large permanent color change onexposure to the typical cure temperatures of the powder compositionsdescribed herein, about 110° C. to 250° C., preferably 120° C. to 200°C. Examples of such pigments include, without limitation, Hansa Red GG12-5000 (Clariant), Novaperm Red HF3S 70 (Clariant), and the like.

In another embodiment, the second powder composition includes one ormore pigments that are critically color-matched to the first powdercomposition. Such compositions include pigments conventionally includedin powder composition, such as White R-900 (duPont), Sicopal YellowL1100 (BASF), Colortherm 10 (Lanxess Corp., Pittsburgh Pa.), and thelike, are heat-stable and do not demonstrate a significant change incolor after the single heating step.

Conventionally, two types of color systems are used to visually observeand assess color changes in pigments included in a coating composition.The color systems have at least three dimensions, in order to includeall possible colors, and can be based either on a specific arrangementof predetermined colors, or by identifying colors mathematically. Themathematical color system is the CIE color system and is based onmathematical description of the light source, objects and a standardobserver. The light reflected or transmitted by an object is measuredwith a spectrophotometer or similar apparatus or instrument. The datacan be mathematically reproduced as three-dimensional CIE color space.Color differences (ΔE) are calculated using the L*a*b* equations, whereL* represents lightness, a* represents redness-greenness and b*represents yellowness-blueness. The quantities on the L*a*b* scale arecalculated using equations known in the art.

The color of a powder composition and the cured coating formed from thecomposition are measured using a coating spectrophotometer. Colordifference ΔL, Δa, and Δb are preferably obtained by subtracting theL*a*b* scale values for the powder composition and the correspondingcured films.

The color change is numerically expressed as the number of units ofcolor short on the L* scale (ΔL), a* scale (Δa), or b* scale (Δb). In anaspect, ΔL ranges from 0 (black) to 100 (white), preferably 0.5 to 20units, more preferably 2 to 15 units. In an aspect, Δa ranges from −60(green) to 60 (red), preferably a shift of 10 to 40 units on the scale,more preferably 15 to 35 units, and Δb ranges from −60 (blue) to 60(yellow), preferably a shift of 5 to 30 units on the scale, morepreferably 10 to 25 units.

Accordingly, in an embodiment, the first and/or second powdercomposition includes one or more pigments that exhibit a large permanentcolor change on curing. The magnitude of the color change may beassessed in a variety of ways known in the art, including preferablyusing the L*a*b* color change system, as described above. In an aspect,the total color change (ΔE) is denoted by a color shift that is easilyobserved by visual or instrumental means, such as with aspectrophotometer, for example. The color shift corresponds to aparticular number of units on at least one axis of the L*a*b* scale.

Accordingly, in an embodiment, where the first and/or second coatingcomposition(s) include(s) one or more pigments that change color attypical cure temperatures of the coating composition(s), the colorchange ΔE corresponds to a shift of at least about 1 unit, preferably atleast about 5 units.

In an embodiment, the methods described herein include applying at leasta second powder composition after at least a first powder compositionhas been applied on the metal substrate. In an aspect, the first powdercomposition is preferably identical or similar in color to the secondpowder composition, i.e. the second powder composition is criticallycolor-matched to the first powder composition.

Accordingly, in an embodiment, the second powder composition iscritically color-matched to the first powder composition. The colormatch of the first and second powder composition may be assessed in avariety of ways known in the art, including preferably using L*a*b*color change system, as described above. In an aspect, where the firstand second coating composition are critically color-matched, the totalcolor change ΔE corresponds to a shift of less than about 2 units,preferably less than about 1 unit, more preferably less than about 0.5units.

Without limiting to theory, it is believed that corrosion resistance,including at the edges, and smoothness of a coating are related to flow.Typically and preferably, edge coverage improves edge corrosionresistance of a coated substrate, and low flow coatings are believed toprovide improved edge coverage, i.e., edge coverage decreases as flowincreases. Conversely, smoothness increases as flow increases. When onlyone powder composition is applied to a substrate, a low flow coatingwill provide good edge coverage but with low surface smoothness. On theother hand, if a high flow composition is used, high surface smoothnessis achieved, but edge coverage is sacrificed. Therefore, in order tocoat a metal substrate to provide optimal edge coverage and smoothness,it is preferable to vary the flow of the first and/or second powdercoating composition.

The flow of a powder coating composition is dependent on variousfactors, including, without limitation, the viscosity of the binderresin(s), oil absorption of extender pigments, flow control agents, theratio of extender pigments to resin used in the composition, thereactivity of the resin, and the like. Conventionally, the flow of apowder coating composition may be adjusted by altering the amount ortype of extender pigment used, by altering the resin or crosslinkerchemistry, or by introducing flow control agents (such as thixotropes,for example). For example, the flow of a powder composition may bereduced by inclusion of extender pigments having oil absorption of atleast about 25 g/100 mL, preferably at least about 35 g/100 mL.

Accordingly, in an embodiment, the first powder composition and secondcomposition are selected based on their relative flow, with flowadjusted or controlled by conventional means. In an aspect, the firstpowder composition is a low flow composition, and the second powdercomposition is a relatively high flow composition. The first powdercomposition has flow of no more than about 40 mm, preferably about 10 to30 mm, more preferably about 15 to 25 mm. In another aspect, the secondpowder composition has flow of at least about 40 mm, preferably morethan about 50 mm, more preferably more than about 70 mm.

Conventionally, substrates are coated with a low flow powder compositionfirst and the coating is heated to melt and cure the composition. Asecond powder composition, typically a high flow composition, is thenapplied over the first coating and melted and cured. This produces acoating with good edge coverage and smoothness, but the process requiresat least two application and heating steps, with a correspondingincrease in production line space, time and energy costs.

In contravention of conventional practice and industry bias, the methodsand systems described herein include steps for sequential application ofa low flow powder composition and a high flow powder composition, butwith a single heating step following the application of the secondcomposition. Surprisingly, the single heating step produces a coatingwith excellent corrosion resistance, including at the edges, and optimalsurface smoothness. In an aspect, the methods described herein produceedge coverage on the order of about 2%, preferably about 5%, morepreferably about 10% of face coverage.

The flow of a coating represents the ability of the coating to wet asubstrate, i.e. when a coating is applied to a substrate, the flow willdetermine how well the coating covers the substrate surface. Theapplication of a coating with poor flow can therefore produce a coatedsurface with a non-uniform or rough texture. Over a given period oftime, the coating will become less rough in a process known as leveling.Flow and leveling have significant influence on the performance andappearance characteristics of a coating. For example, inadequate flow ofa coating leads to defects such as craters and pinholes as a result ofthe coating not completely covering the substrate. Incomplete coverageof the substrate as a result of inadequate flow also leaves parts of thesubstrate exposed and prone to corrosion, thereby affecting performance.

The flow and leveling of an applied powder coating are governed byviscosity and surface tension, with the appearance of a coatingdetermined largely by viscosity and by the time available for leveling.The viscosity of an uncured powder compositions changes during cure.Initially, a powder composition will melt and flow over the substrate asthe temperature increases beyond the glass transition temperature (Tg)of the composition and until the composition reaches its minimumviscosity. When the composition begins to cure, the viscosity willincrease and therefore, the ability of the powder to flow and cover thesubstrate surface will decrease. The viscosity of a composition relatesto its reactivity, and without limiting to theory, a highly reactivecomposition may never reach its minimum viscosity and therefore neverdemonstrate adequate flow. Therefore, it is important to balanceviscosity and the reactivity of a composition in order to control flow.Conventionally, the flow of a powder is lowest when the minimumviscosity of the powder composition is highest. The high minimumviscosity limits the composition from flowing over the surface, and ifnot optimized, leads to voids or pinholes in the cured coating.

Without limiting to theory, it is believed that surface tension affectsthe ability of a powder composition to wet a substrate. If surfacetension is too high, poor wetting occurs, and defects such as cratersare formed where the powder cannot flow over the substrate. On the otherhand, if surface tension is too low, other defects such as sagging andpoor edge coverage will be seen. Without limiting to theory, surfacetension and viscosity also influence the leveling behavior of acomposition. Therefore, to obtain a coating with optimized flow andleveling, acceptable levels of air entrainment and final appearance, itis necessary to balance the viscosity, surface tension and reactivity orcure behavior of the composition.

Conventionally, when two or more powder coating compositions are appliedto a substrate, each layer is cured individually before the next layeris applied. As a result, it is possible to control the flow and levelingof each layer independently in order to obtain a coating with optimalappearance and performance characteristics. However, in the methodsdescribed herein, two or more layers of powder composition are appliedto a substrate with only a single curing step, and therefore, therheological and kinetic properties of the coating composition must becontrolled to provide optimal appearance and performancecharacteristics. Accordingly, the powder coating compositions describedherein have been modified to have a combination of leveling, viscosityand reactivity that leads to a system with optimal performance inenvironmental testing, mechanical testing, and appearance.

Conventionally, for a low flow product, a binder resin with a highviscosity (i.e. at least 3000 Pa, preferably about 3000 to 5000 Pa-s) isused along with flow control agents to adjust the viscosity of theultimate coating composition. However, such low flow compositions oftendemonstrate poor wetting and outgassing relative to conventional highflow compositions. In contravention of conventional practice, themethods and systems described herein use a low flow coating compositionwith low viscosity for the first coating composition. Surprisingly, thelow viscosity composition produces a coating with excellent flow andwetting characteristics, i.e. comparable to a conventional high flowcomposition, with minimal film or surface defects. Furthermore, the lowflow, low viscosity compositions described herein produce filmseffective in outgassing, comparable to conventional high flowcompositions.

Accordingly, in an embodiment, the first coating composition is a lowflow composition, and the second powder composition is a relatively highflow composition. The first powder composition has a viscosity of almostfour times greater than the viscosity of the second composition. Thisdifference in viscosities will provide optimal edge coverage along withacceptable surface flow and leveling for the first coating, while thehigh flow of the second coating composition will produce a high glosssmooth surface.

The flow and leveling characteristics of the first and secondcompositions must therefore be carefully balanced to obtain a powdercoating with optimal performance and aesthetic appearance, asdemonstrated by the flow index shown in FIG. 1. However, it is notpossible to obtain a coating with the required performance or appearanceby simply blending the first and second compositions, as such a blendwill tend to have the flow characteristics of the component present athigher concentration. For example, as seen in FIG. 1, a blend of a lowflow composition (15%) and a high flow composition (85%) will have flowindex similar to a high flow composition, and therefore, the blend willnot have the necessary performance characteristics.

The coating as described herein is formed only when a first and secondcoating composition are independently applied to a substrate and curedsimultaneously. This process of applying two coatings with a single curestep provides unique challenges with respect to flow and leveling. Forinstance, if the first composition has a high viscosity and slow curebehavior, any entrapped air will take a longer time to leave thecoating. If the second composition has low viscosity and fast curebehavior, the low viscosity helps the second composition removeentrapped air. Where only a single curing step is used and the first andsecond coatings are cured simultaneously, the coating will include afirst layer that is not cured with a second layer that is fully cured,and entrapped air will be present between the layers. Therefore,controlling the flow, leveling and cure characteristics of eachcomposition is of critical importance.

In an aspect, the methods described herein produce optimal surfacesmoothness. The methods described herein produce surface smoothness onthe PCI scale of at least 4, preferably at least 5. Measured as20-degree gloss, using the method set out in ASTM D523, the methodsdescribed herein produce surface smoothness of about 25 to 90%,preferably above 60% of the specular reflectance of a reference blackglass standard. Typically and preferably, the smoothness of the surfacewill be determined by the desired end use for the powder-coated metalsubstrate.

In an embodiment, the first or second powder composition includes atleast one polymeric binder. The powder composition may also optionallyinclude one or more pigments, opacifying agents or other additives.

Suitable polymeric binders generally include a film forming resin andoptionally a curing agent for the resin. The binder may be selected fromany resin or combination of resins that provides the desired filmproperties. Suitable examples of polymeric binders include thermosetand/or thermoplastic materials, and can be made with epoxy, polyester,polyurethane, polyamide, acrylic, polyvinylchloride, nylon,fluoropolymer, silicone, other resins, or combinations thereof.Thermoset materials are preferred for use as polymeric binders in powdercoating applications, and epoxies, polyesters and acrylics arepreferred. If desired, elastomeric resins may be used for certainapplications. In an aspect, specific polymeric binders or resins areincluded in the powder compositions described herein depending on thedesired end use of the powder-coated substrate. For example, certainhigh molecular weight polyesters show superior corrosion resistance andare suitable for use on substrates used for interior and exteriorapplications.

In an aspect, the first and second powder compositions include the samepolymeric binder. In another aspect, the first and second powdercompositions include different polymeric binders. Examples of preferredbinders include the following: carboxyl-functional polyester resinscured with epoxide-functional compounds (e.g., triglycidyl-isocyanurateor TGIC), carboxyl-functional polyester resins cured with polymericepoxy resins, carboxyl-functional polyester resins cured withhydroxyalkyl amides (HAA), hydroxyalkyl urea (HAU), hydroxyl-functionalpolyester resins cured with blocked isocyanates or uretdiones, epoxyresins cured with amines (e.g., dicyandiamide), epoxy resins cured withphenolic-functional resins, epoxy resins cured with carboxyl-functionalcuratives, carboxyl-functional acrylic resins cured with polymeric epoxyresins, hydroxyl-functional acrylic resins cured with blockedisocyanates or uretdiones, unsaturated resins cured through free radicalreactions, and silicone resins used either as the sole binder or incombination with organic resins. The optional curing reaction may beinduced thermally, or by exposure to radiation (e.g., UV, UV-vis,visible light, IR, near-IR, and e-beam).

In an embodiment, the powder compositions are heated to the typical cureor melt temperatures of the compositions, i.e. about 110° C. to about250° C., preferably 120° C. to 200° C.

In order for a powder coating composition to be effective, thecomposition must be resistant to sintering or substantiallynon-sintering, i.e. the powder composition must retain its particulatecharacteristics even when exposed to specific conditions. The sinteringresistance of a powder composition is typically maintained by usingcompositions having a Tg of 45° C. or higher. Conventionally, thesecompositions are cured at temperatures of about 180° C. to about 250°C., or even at lower temperatures of about 140° C. to 170° C. However,high Tg compositions may not demonstrate optimum coalescing or levelingwhen cured at reduced temperatures less than about 140° C., resulting inpoor film formation and inadequate mechanical properties.Conventionally, therefore, powder coatings which are intended forreduced temperature cure are generally formulated with resins havingreduced Tg, resulting in increased tendency for the powder coating tosinter and create lumps during storage. Without limiting to theory, byselecting polymeric binder resins with transition temperature (Tg) of atleast 50° C., more preferably about 55° C. to 70° C., and mostpreferably about 60° C. to 65° C., a powder coating composition capableof cure at low temperatures of 120° C. to 135° C. can be made withoutany problems with coalescing or sintering typically expected at high Tg.Such coatings are described as ultra-low cure powder compositions, andare described in Applicant's Application No. PCT/US2013/025302, filedFeb. 8, 2013, now published as WO2014123534.

The first or second powder composition may include other additives.These other additives can improve the application of the powder coating,the melting and/or curing of that coating, or the performance orappearance of the final coating. Examples of optional additives whichmay be useful in the powder include: cure catalysts, antioxidants, colorstabilizers, slip and mar additives, UV absorbers, hindered amine lightstabilizers, photoinitiators, conductivity additives, tribochargingadditives, anti-corrosion additives, fillers, texture agents, degassingadditives, flow control agents, thixotropes, and edge coverageadditives.

The polymeric binder is dry mixed together with any additives, and thenis typically melt blended by passing through an extruder. The resultingextrudate is solidified by cooling, and then ground or pulverized toform a powder. Other methods may also be used. For example, onealternative method uses a binder that is soluble in liquid carbondioxide. In that method, the dry ingredients are mixed into the liquidcarbon dioxide and then sprayed to form the powder particles. Ifdesired, powders may be classified or sieved to achieve a desiredparticle size and/or distribution of particle sizes.

The resulting powder is at a size that can effectively be used by theapplication process. Practically, particles less than 10 microns in sizeare difficult to apply effectively using conventional electrostaticspraying methods. Consequently, powders having median particle size lessthan about 25 microns are difficult to electrostatically spray becausethose powders typically have a large fraction of small particles.Preferably the grinding is adjusted (or sieving or classifying isperformed) to achieve a powder median particle size of about 25 to 150microns, more preferably 30 to 70 microns, most preferably 30 to 50microns.

Optionally, other additives may be used in the present invention. Asdiscussed above, additives may be added prior to extrusion and be partof the base powder, or may be added after extrusion. Suitable additivesfor addition after extrusion include materials that would not performwell if they were added prior to extrusion; materials that would causeadditional wear on the extrusion equipment, or other additives.

Additionally, optional additives include materials which are feasible toadd during the extrusion process, but may also be added later. Theadditives may be added alone or in combination with other additives toprovide a desired effect on the powder finish or the powder composition.These other additives can improve the application of the powder, themelting and/or curing, or the final performance or appearance. Examplesof optional additives which may be useful include: cure catalysts,antioxidants, color stabilizers, slip and mar additives, UV absorbers,hindered amine light stabilizers, photoinitiators, conductivityadditives, tribocharging additives, anti-corrosion additives, fillers,texture agents, degassing additives, flow control agents, and the like.

In a preferred embodiment, the compositions described herein includeadditives that improve the electrostatic application characteristics ofthe powder coating compositions. Suitable additives of this typeinclude, for example, extrudable application additives, fumed metaloxides, combinations thereof, and the like. In an aspect, theapplication additive is added to the raw material before extrusion, andother additives such as the metal oxide, for example, can be addedlater, during grinding or pulverization of the composition. Additives ofthis type are further described in Applicant's co-pending InternationalApplication No. PCT/US2013/030506, filed Mar. 12, 2013.

Other preferred additives include performance additives such asrubberizers, friction reducers, and microcapsules. Additionally, theadditive could be an abrasive, a heat sensitive catalyst, an agent thathelps create a porous final coating, or that improves wetting of thepowder.

Techniques for preparing low flow and high flow powder compositions areknown to those of skill in the art. Mixing can be carried out by anyavailable mechanical mixer or by manual mixing. Some examples ofpossible mixers include Henschel mixers (available, for example, fromHenschel Mixing Technology, Green Bay, Wis.), Mixaco mixers (availablefrom, for example, Triad Sales, Greer, S.C. or Dr. Herfeld GmbH,Neuenrade, Germany), Marion mixers (available from, for example, MarionMixers, Inc., 3575 3rd Avenue, Marion, Iowa), invertible mixers,Littleford mixers (from Littleford Day, Inc.), horizontal shaft mixersand ball mills. Preferred mixers would include those that are mosteasily cleaned.

Powder coatings are generally manufactured in a multi-step process.Various ingredients, which may include resins, curing agents, pigments,additives, and fillers, are dry-blended to form a premix. This premix isthen fed into an extruder, which uses a combination of heat, pressure,and shear to melt fusible ingredients and to thoroughly mix all theingredients. The extrudate is cooled to a friable solid, and then groundinto a powder. Depending on the desired coating end use, the grindingconditions are typically adjusted to achieve a powder median particlesize of about 25 to 150 microns.

The final powder may then be applied to an article by various meansincluding the use of fluid beds and spray applicators. Most commonly, anelectrostatic spraying process is used, wherein the particles areelectrostatically charged and sprayed onto an article that has beengrounded so that the powder particles are attracted to and cling to thearticle. Typically, a corona charging process is used for electrostaticapplication, although tribo charging or a combination of corona andtribo charging may be used. After coating, the article is heated. Thisheating step causes the powder particles to melt and flow together tocoat the article. Optionally, continued or additional heating may beused to cure the coating. Other alternatives such as UV curing of thecoating may be used.

The coating is optionally cured, and such curing may occur via continuedheating, subsequent heating, or residual heat in the substrate. Inanother embodiment of the invention, if a radiation curable powdercoating base is selected, the powder can be melted by a relatively shortor low temperature heating cycle, and then may be exposed to radiationto initiate the curing process. One example of this embodiment is aUV-curable powder. Other examples of radiation curing include usingUV-vis, visible light, near-IR, IR, and e-beam.

The compositions and methods described herein may be used with a widevariety of substrates. Typically and preferably, the powder coatingcompositions described herein are used to coat metal substrates,including without limitation, unprimed metal, clean-blasted metal, andpretreated metal, including plated substrates and ecoat-treated metalsubstrates. Typical pretreatments for metal substrates include, forexample, treatment with iron phosphate, zinc phosphate, and the like.Metal substrates can be cleaned and pretreated using a variety ofstandard processes known in the industry. Examples include, withoutlimitation, iron phosphating, zinc phosphating, nanoceramic treatments,various ambient temperature pretreatments, zirconium containingpretreatments, acid pickling, or any other method known in the art toyield a clean, contaminant-free surface on a substrate.

The coating compositions and methods described herein may be, but arenot limited to conversion coatings, i.e. parts or surfaces where thesubstrate is converted into a coating by a chemical or electrochemicalprocess. The coating compositions described herein may be applied tosubstrates previously coated by various processes known to persons ofskill in the art, including for example, ecoat methods, plating methods,and the like. For example, for many applications, the substrate may bepretreated and then coated by an electrocoat process to produce a primedsubstrate. The powder coating compositions described herein are thenapplied over the ecoat-primed substrate.

There is no expectation that substrates to be coated with thecompositions described herein will always be bare or unprimed metalsubstrates. In an embodiment, the methods described herein may be usedto spray the entire substrate. In another embodiment, the methodsdescribed herein may be used to spray only the edges of the substrate.Preferably, the coated substrate has desirable physical and mechanicalproperties, including optimal edge coverage of sharp edges and surfacesmoothness. Typically, the final film coating will have a thickness of20 to 200 microns, preferably 75 to 150 microns.

The following examples are offered to aid in understanding of thepresent invention and are not to be construed as limiting the scopethereof. Unless otherwise indicated, all parts and percentages are byweight.

EXAMPLES

Unless indicated otherwise, the following test methods were utilized inthe Example(s) that follow(s).

Melt Flow Measurement

The melt flow of the powder compositions is tested using ASTM D3451(Standard Guide for Testing Coating Powders and Powder Coatings).

Edge Coverage

The edge coverage of the powder coatings is tested using the methoddescribed in ASTM D2967 (Standard Test Method for Corner Coverage ofPowder Coatings).

Smoothness

The surface smoothness of the coating is measured as 20-degree glossusing the procedure described ASTM D523 (Standard Test Method forSpecular Gloss).

Example 1 Comparison of Coating Types

Powder compositions were prepared as indicated in Table 1 and applied to0.020-in (0.05 cm) thick cold-rolled steel panels. The total coatingthickness for each coating in Table 1 (whether single-layer ordual-layer) was about 75-90 microns (3.0 to 3.6 mil). Flow, edgecoverage, and smoothness for each coating type were then measured.

TABLE 1 Flow Edge 20-degree Type Coating (mm) Coverage (%) Gloss (%)Composition #1 Low 21 14 45 Flow Composition #2 High 78 1.0 92 FlowComparative (low flow primer Two cure N/A 6.6 85 and high flow topcoat)Inventive (low flow primer and Single N/A 2.4 81 high flow topcoat) cure

Example 2 Edge Coverage and Smoothness as Function of Flow

Panels were prepared as described in Example 1, except that only asingle powder composition is applied, with the composition selectedaccording to the flow shown in Table 2. Edge coverage and smoothness arethen determined for each panel.

TABLE 2 Coating Flow (mm) Edge Coverage (%) 20-degree Gloss (%) 19 21.827 21 14.3 45 23 12.2 50 29 7.4 58 34 5.5 57 39 2.9 74 78 1.0 92

As can be seen from Table 2, a coating with a flow of less than about 40provides acceptable edge coverage while a coating with a flow of aboveabout 40 provides optimal surface smoothness.

Example 3 Preparation and Measurement of Colored Powder CoatingCompositions

Commercially available powder coatings (#1 to #9) were obtained (ValsparCorp., Minneapolis Minn.). In addition, powder coating compositions wereprepared as indicated in Table 5 (#10 and #11; comparative) and Table 6(#12 to #15; exemplary). Each composition included 850 parts by weightD.E.R. 6224 epoxy resin (Dow Chemical Co., Midland Mich.), 150 parts ofan imidazole-catalyzed phenolic curing agent KD-404J (Kukdo ChemicalCo., Seoul, Korea), and 18 parts of flow control agent Resiflow PF-67(Estron Chemical Co., Calvert City Ky.). In addition, each compositionincluded pigments of the type and amount shown in Tables 1 and 2. Thecomponents of each composition were blended in plastic bags,melt-blended through a twin-screw extruder and cooled to solidify. Thecooled compositions were then ground to a coating composition, withoversized particles removed by sieving through an 80-mesh (0.0070 in)screen. For each composition, color was measured by pouring thecomposition at least 0.1 in deep on a steel panel. Clear colorlesspacking tape was applied to confine the powder composition to ablister-shaped area. The color of the powder blister was measured usinga spectrophotometer.

TABLE 3 Comparative Compositions Pigment #10 (parts by weight) #11(parts by weight) White R-900 100 100 Yellow L1100 0 10

TABLE 4 Exemplary Compositions Pigment #12 #13 #14 #15 White R-900 10050 100 41 Yellow L1100 0 30 0 1.4 Colortherm 10 0 0 0 94 Hansa Red GG 1020 0 0 Novaperm Red HF3S 70 0 0 10 10

Example 4 Preparation and Measurement of Coating Films

The powder compositions of Example 3 were applied to 0.32 in-thickgrounded steel panels using electrostatic spray methods. Coated panelswere cured at 400° F. and allowed to cool to room temperature. Theresultant coating films were 2.8 to 3.2 mil (0.07 to 0.08 mm) inthickness. The color of the cured films was measured using a coatingspectrophotometer. Color differences ΔL, Δa, and Δb were obtained bysubtracting measured color values for powders and corresponding curedfilms. Results are shown in Tables 3 and 4.

TABLE 5 Color Change for Comparative Compositions Numerical Color ChangeVisual Color Change (Powder to Film) Powder Film ΔL Δa Δb 1 Bright BlueDark Blue −12.2 11.1 8.5 2 Black Black −2.7 −0.5 −1.1 3 White White −1.5−0.1 −2.1 4 Beige Beige −0.4 0.3 −0.4 5 Yellow Yellow −3.3 0.7 −6.4 6Orange Orange −5.7 −3.4 −7.0 7 Red Red −3.5 −11.1 −2.5 8 Bright DarkGreen −9.9 11.4 −5.2 Green 9 Brown- Brown- −4.8 0.7 2.17 shade shadeYellow Yellow 10 White Off-white −9.7 −1.3 6.9 11 Yellow Yellow −7.8 1.8−2.0

TABLE 6 Color Change for Exemplary Compositions Numerical Color ChangeVisual Color Change (Powder to Film) Example Powder Film ΔL Δa Δb 12Orange Off-white 4.0 −31.6 −11.4 13 Orange Yellow −2.5 −35.0 −9.5 14 RedOff-white 12.1 −37.7 7.7 15 Orange Brown- 0.9 −15.0 6.1 shade Yellow

Example 5 Preparation and Measurement of Powder Rheological Behaviorwith Temperature

A representative sample of a primer and topcoat formulation (as shown inTable 5, for example) were each examined to compare their rheologicalbehavior, with a blend of the topcoat and primer formulation (85%topcoat, 15% primer) used for comparison. The time and temperaturedependence of the viscosity was measured on a strain-controlledrheometer using parallel plate geometry in dynamic mode, with an appliedstrain of 1% and 1 Hz frequency of oscillation. The data is representedas complex viscosity (Pa-s). For the primer and topcoat formulations,sample discs were prepared by compressing 0.3 g of the powdercomposition in a 13-mm plug mold at 1000 ram force for 15 seconds.Pressing was done at room temperature to avoid premature curing of thepowder. The rheometer was set at 70° C. for sample loading. The gap wasthen set to 0.05 mm below the thickness of the pressed pellet to ensurecontact with parallel plates. The temperature of the sample and fixtureswere equilibrated for 10 minutes before measurements were taken. Thesample temperature was ramped at 5° C./min from 70° C. to 245° C. toexamine the rheological behavior as a function of temperature. Theresultant curve of complex viscosity versus temperature is shown inFIGS. 2A and 2B, and is examined to compare the flow behavior of thepowder compositions.

1-20. (canceled)
 21. A coated article made by a process comprising:providing a metal substrate; providing at least a first powder coatingcomposition with flow of no more than about 40 mm to a surface of themetal substrate; providing at least a second powder coating compositionwith flow of at least about 40 mm; and heating the first powder coatingand second powder coating composition in a single step to form acontinuous cured coating on the metal substrate, thereby forming thecoated article, wherein the cured coating is corrosion-resistant andhaving a surface smoothness on the Powder Coating Institute (PCI) scaleof at least
 4. 22. The article of claim 21, wherein the first powdercoating composition has a flow of about 15 mm to 40 mm.
 23. The articleof claim 21, wherein the first powder coating composition has a flow ofabout 20 to 35 mm.
 24. The article of claim 21, wherein the secondpowder coating composition has flow of greater than about 50 mm.
 25. Thearticle of claim 21, wherein the second powder coating composition hasflow of greater than about 70 mm.
 26. The article of claim 21, whereinthe second powder coating composition has flow of greater than about 75mm.
 27. The article of claim 21, wherein the cured coating has edgecoverage equal to at least 2% of face coverage.
 28. The article of claim21, wherein the cured coating has 20-degree gloss of at least 50%. 29.The article of claim 21, wherein the cured coating has edge coverageequal to about 10% of the face coverage.
 30. The article of claim 21,wherein the metal substrate is unprimed, clean blasted, or pretreated.31. The article of claim 21, wherein the metal substrate may be heatedprior to applying the first powder coating composition.
 32. The articleof claim 21, wherein the first powder coating composition is applied tothe metal substrate at ambient temperature.
 33. The article of claim 21,wherein the first powder coating composition and second powder coatingcomposition are heated in a single step at a temperature of about 180°C. to 200° C.
 34. The article of claim 21, wherein the first powdercoating composition and second coating composition are heated in asingle step at a temperature of about 140° C. to 170° C.
 35. The articleof claim 21, wherein the first powder coating composition and secondcoating composition are heated in a single step at a temperature ofabout 120° C. to 135° C.
 36. The article of claim 21, wherein the firstpowder coating composition is applied only to one or more edges of thesubstrate.
 37. The article of any claim 21, wherein the second powdercoating composition is applied only to one or more edges of thesubstrate.
 38. The article of claim 21, wherein the first powder coatingcomposition and the second coating composition are applied only to oneor more edges of the substrate.
 39. The article of claim 21, wherein thefirst powder coating composition is applied at a film build of about 20to 40 micron.
 40. The article of claim 21, wherein the second powdercoating composition is applied at a film build of about 25 to 35 micron.